The main reason why "physical punishment/discipline" is illegal is because not many people know where to stop and (like Snowy suggested) some children react differently to it.
My parents ALWAYS explained why they hit me after the fact in a calm and controlled manner.
Sticking my finger into a socket? *smack* Don't do that, you will hurt yourself
Touching stuff at other people's houses? *smack* Don't do that, it is rude and you might break something
Said a bad word? *mouth covered* Don't say that, it is not polite
Whether I understood it at the age or not, giving an explanation I believe just removes the confusion as to why the kid was disciplined.
I also think age should be taken into account as well as CONSISTENCY across parents. If the mother is being the main discipliner and the father is lax about behaviour. The children will start to favour the father and hold resentment against the mother and vice versa.
After a while of smack + explanation, the soon to be 10+ year old child will just associate a stern explanation to be a fair amount of discipline. If you're still smacking your 15 year old then, uhh...
Rant: Feeling a bit annoyed at people who are taking FOREVER to RSVP for a party even after I've asked them several times to do so :I